A known microcrystalline diamond (hereinafter, referred to as “MCD”) thin film synthesis apparatus is used to synthesize a nanocrystalline diamond (hereinafter, referred to as “NCD”) thin film. In this case, however, the synthesis parameters for the NCD thin file are employed to be distinct from the synthesis parameters for the MCD thin film. Due to the distinct synthesis parameters employed, stability and uniformity may be markedly deteriorated. Accordingly, the known synthesis apparatus and process need to be considerably changed.
A microwave plasma CVD method or a hot filament CVD method is hitherto representatively used to deposit the MCD thin film and the NCD thin film. The microwave plasma CVD method has problems in that it is hard to form a large-area thin film and to handle a microwave. In the hot filament method, a simple thermal decomposition method is used. Accordingly, the hot filament method is economic in view of ease of forming a large-area thin film and a simple structure. However, since a filament is used, inconvenience may be caused, and impurities may be introduced. In addition, the thermal decomposition method has problems against the plasma method. Therefore, there is a need for a new synthesis method to overcome the drawbacks inherent in the related art.